The present invention relates to oscillators and, more particularly, to LC balanced oscillators.
Prior art LC balanced oscillators are well known to those skilled in the art wherein the center oscillation frequency is determined by a LC tank circuit. The prior art is replete with systems which utilize such oscillators. For example, FM receivers and television modulators use LC balanced oscillators as the local oscillators. The MC1732, an integrated circuit manufactured by Motorola Inc., is an example of a television video modulator circuit which uses a LC balanced oscillator to generate a video output signal. In this case the LC components are externally coupled to the internal oscillator components. The oscillator drives a balanced multiplier which is used as a mixer or modulator. An advantage of such balanced systems is that the oscillator signal is removed from the multiplier output.
Typically, the oscillator comprises a pair of transistors the emitters of which are commonly connected to a source of constant current. The base electrodes of the two transistors are cross-connected to the collector of the opposite transistor. The collectors are each coupled to a voltage reference source. The LC tank is coupled across the collectors of the two transistors. The output of the oscillator is taken differentially across the bases of the two transistors. A balanced multiplier comprising a pair of transistors having their emitters commonly connected to a source of current is driven by the oscillator with the differential output of the latter being connected across the base electrodes of the two transistors of the multiplier.
Although prior art LC balanced oscillators of the type described above work well they suffer a major problem. Any signal voltage appearing at the commonly connected emitters of the oscillator will appear at the commonly connected emitters of the transistors of the balanced multiplier. This signal will then be converted to a common mode current which flows through the two transistors. Thus, the oscillator signal developed across the tank in the prior art oscillator, assuming equal collector resistors, will produce a full wave rectified signal at twice the oscillator frequency at the emitters of the transistors of both the oscillator and multiplier. This common mode signal is highly undesirable as it can produce distortion in the modulator output signal and can decrease the conversion gain of the system.
Hence, a need exists for an improved LC balanced oscillator which eliminates or severely restricts common mode signal problems associated with some prior art oscillators.